Systems and methods for adaptive bit loading in a multiple antenna orthogonal frequency division multiplexed communication system

ABSTRACT

In an orthogonal frequency division multiplexed (OFDM) system, a transmitter and/or receiver communicate separate data streams on non-orthogonal spatial channels. Each spatial channel may use the same set of OFDM subcarriers and may take advantage of the multipath characteristics of the spatial channel allowing the communication of additional data without an increase in frequency bandwidth. Space-frequency subcarrier modulation assignments may be dynamically assigned on a per subcarrier basis as well as a per spatial channel basis to help maximize the data-carrying capacity of the channel. In some embodiments, each of the spatial channels may be associated with one of a plurality of spatially diverse antennas. In other embodiments, beamforming may be performed to allow the transmission and/or reception of signals within the spatial channels.

This application is a continuation of U.S. patent application Ser. No.10/750,587, filed on Dec. 29, 2003, which claims the benefit of U.S.Provisional Patent Application Ser. No. 60/493,937, filed on Aug. 8,2003, both of which are incorporated herein by reference in theirentireties.

TECHNICAL FIELD

Embodiments of the present invention pertain to wireless communications.In some embodiments, the present invention pertains to orthogonalfrequency division multiplexed communications, and in some embodiments,the present invention pertains to wireless local area networks.

BACKGROUND

The data rate of many conventional orthogonal frequency divisionmultiplexed (OFDM) systems is limited by a maximum modulation order(e.g., bits per symbol) that may be effectively communicated on thesymbol-modulated subcarriers of an OFDM channel. Thus, there are generalneeds for apparatus and methods for communicating additional datawithout an increase in frequency bandwidth.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended claims are directed to some of the various embodiments ofthe present invention. However, the detailed description presents a morecomplete understanding of embodiments of the present invention whenconsidered in connection with the figures, wherein like referencenumbers refer to similar items throughout the figures and:

FIG. 1 is a block diagram of a transmitter in accordance with someembodiments of the present invention;

FIG. 2 is a block diagram of a receiver in accordance with someembodiments of the present invention;

FIG. 3 is a block diagram of a wireless communication device inaccordance with some embodiments of the present invention;

FIG. 4 is a flow chart of an OFDM symbol transmission procedure inaccordance with some embodiments of the present invention; and

FIG. 5 is a flow chart of an OFDM symbol reception procedure inaccordance with some embodiments of the present invention.

DETAILED DESCRIPTION

The following description and the drawings illustrate specificembodiments of the invention sufficiently to enable those skilled in theart to practice them. Other embodiments may incorporate structural,logical, electrical, process, and other changes. Examples merely typifypossible variations. Individual components and functions are optionalunless explicitly required, and the sequence of operations may vary.Portions and features of some embodiments may be included in orsubstituted for those of others. The scope of embodiments of theinvention encompasses the full ambit of the claims and all availableequivalents of those claims. Such embodiments of the invention may bereferred to, individually or collectively, herein by the term“invention” merely for convenience and without intending to voluntarilylimit the scope of this application to any single invention or inventiveconcept if more than one is in fact disclosed.

FIG. 1 is a block diagram of a transmitter in accordance with someembodiments of the present invention. Transmitter 100 may be part of awireless communication device, and may transmit orthogonal frequencydivision multiplexed (e.g., OFDM) communication signals. Transmitter 100may transmit an OFDM symbol on a communication channel within apredetermined frequency spectrum. The channels may comprise a pluralityof orthogonal subcarriers. In some embodiments, the orthogonalsubcarriers of a channel may be closely spaced OFDM subcarriers. Toachieve orthogonality between the closely spaced subcarriers, thesubcarriers of a particular channel may have null at substantially acenter frequency of the other subcarriers of that channel.

In some embodiments, transmitter 100 may utilize more than one ofspatially-diverse antennas 114 to “divide” the channel into one or morespatial channels. In some embodiments, each transmit antenna 114 maydefine one spatial channel. In other embodiments, beamforming may beused to “divide” the channel into spatial channels. In theseembodiments, each spatial channel may be used to communicate separate orindependent data streams on the same subcarriers as the other spatialchannels, allowing the communication of additional data without anincrease in frequency bandwidth. The use of spatial channels may takeadvantage of the multipath characteristics of the channel. In someembodiments, the spatial channels may be non-orthogonal channels,although the scope of the invention is not limited in this respect.

In accordance with some embodiments, transmitter 100 may individuallysymbol-modulate the subcarriers of each spatial channel in accordancewith individual subcarrier modulation assignments. This may be referredto as adaptive bit loading (ABL). Accordingly, one or more bits may berepresented by a symbol-modulated on a subcarrier. The modulationassignments for each spatial channel may be based on the channelcharacteristics or channel conditions for that spatial channel, althoughthe scope of the invention is not limited in this respect. In someembodiments, these spatial-frequency subcarrier modulation assignmentsmay range from zero bits per symbol to up to ten or more bits persymbol. In terms of modulation levels, the spatial-frequency subcarriermodulation assignments may comprise binary phase shift keying (BPSK),which communicates one bit per symbol, quadrature phase shift keying(QPSK), which communicates two bits per symbol, 8PSK, which communicatesthree bits per symbol, 16-quadrature amplitude modulation (16-QAM),which communicates four bits per symbol, 32-QAM, which communicates fivebits per symbol, 64-QAM, which communicates six bits per symbol,128-QAM, which communicates seven bits per symbol, and 256-QAM, whichcommunicates eight bits per symbol. Subcarrier modulation assignmentswith higher data communication rates per subcarrier (e.g., ten bits) mayalso be used.

An OFDM symbol may be viewed as the combination of the symbols modulatedon the individual subcarriers of all the spatial channels. Because ofthe variable number of bits per symbol-modulated subcarrier and thevariable number of spatial channels that may be used, the number of bitsper OFDM symbol may vary greatly. For example, when four transmitantennas are used to provide four spatial channels, when each spatialchannel uses up to 48 OFDM data subcarriers, and when each subcarrierhas a spatial-frequency subcarrier modulation assignment ranging betweenzero and six bits per symbol, the number of bits per OFDM symbol mayrange up to 1152 bits (4 spatial channels×48 data subcarriers perspatial channel×6 bits per symbol), depending on the channel conditionsof the spatial channels, among other things.

In accordance with some embodiments, data for transmission over thespatial channels is provided to transmitter 100 in the form of bitstream 101. Encoder 102 may apply forward error correcting (FEC) codesto bit stream 101 to generate coded bits comprising bit stream 103. Bitinterleaver 104 may perform an interleaving operation on a block of bitsto generate interleaved block of bits 105. Block of bits 105 mayrepresent an OFDM symbol. Parser 106 may parse block of bits 105 intogroups of bits 107 having a variable number of coded bits. The variablenumber of coded bits of a group may be determined by thespatial-frequency subcarrier modulation assignments associated with aparticular subcarrier of a particular spatial channel. Subcarriermodulators 108 may individually modulate the groups of bits 107 oncorresponding OFDM subcarriers for each spatial channel in accordancewith the spatial-frequency subcarrier modulation assignments to generatesymbol-modulated subcarriers 109. In some embodiments, parser 106 mayinclude a serial-to-parallel conversion to provide the groups of bits ina parallel form to subcarrier modulators 108.

In some embodiments, symbol-modulated subcarriers 109 may comprise asymbol-modulated subcarrier for each subcarrier of a spatial channel. AnOFDM symbol may be represented by the combination of allsymbol-modulated subcarriers 109. In some of these embodiments, aplurality of individual subcarrier modulators 108 (e.g., one for eachsubcarrier) may each separately modulate an individual OFDM subcarrier.In these embodiments, each one of subcarrier modulators 108 may modulatesymbols for the same frequency subcarrier of the different spatialchannels.

Inverse Fast Fourier transform (IFFT) circuitry 110 may perform IFFTs onsymbol-modulated subcarriers 109 to generate time domain representationsof the OFDM symbol. Almost any form of inverse discrete Fouriertransform (IDFT) may be used to perform the inverse transform operation.The number of time domain samples generated by IFFT circuitry 110 may beequal to the number of frequency components input thereto. In someembodiments, IFFT circuitry 110 may generate a time domain waveform foreach spatial channel from the combination of symbol-modulatedsubcarriers 109 for that spatial channel.

IFFT circuitry 110 may also convert the time domain samples generated bythe IFFT operation, which may be in a parallel form, to one or moreserial symbol streams 111. IFFT circuitry 110 may also add a cyclicextension (or guard interval) to reduce inter-symbol interference in thechannel. In some embodiments, the number of serial symbol streams 111generated by IFFT circuitry may correspond to the number of spatialchannels, although the scope of the invention is not limited in thisrespect. Radio frequency (RF) circuitry 112 may prepare each of serialsymbol streams 111 for RF transmission over a corresponding one of thespatial channels.

In some embodiments, each of spatially diverse antennas 114 may beassociated with a spatial channel and may receive RF signals from anassociated one of RF circuitry 112. Spatially diverse antennas 114 maybe separated by a distance. A minimum separation distance may be basedon the wavelength of the frequency spectrum used for communicating. Insome embodiments, a separation of a few centimeters may be sufficient tohelp assure multipath differences between the spatial channels. Antennas114 may comprise one or more directional or omnidirectional antennas,including, for example, dipole antennas, monopole antennas, loopantennas, microstrip antennas or other types of antennas suitable fortransmission of RF signals by transmitter 100.

In some embodiments, the spatial-frequency subcarrier modulationassignments may be based on channel conditions, such as a signal tointerference and noise ratio (SINR) for the particular subcarrier in aparticular spatial channel. In some embodiments, the spatial-frequencysubcarrier modulation assignments may be determined and provided by areceiving station, although the scope of the invention is not limited inthis respect. In some embodiments, higher subcarrier modulationassignments (e.g., more bits per symbol) may be used for subcarriershaving better SINRs.

In some embodiments, bit interleaver 104 may input a variable number ofcoded bits of bit stream 103 into an interleaver matrix of interleaver104. In some embodiments, the variable number of coded bits may compriseone OFDM symbol and may comprise the number of coded bits per OFDMsymbol (Ncbps). In some embodiments, system controller 118 may calculatethe variable number of coded bits per OFDM symbol based on thespatial-frequency subcarrier modulation assignments for the subcarriersfor each spatial channel. In some embodiments, the number of coded bitsper OFDM symbol (Ncbps) may be provided by system controller 118 to bitinterleaver 104.

In some embodiments, system controller 118 may generate and providetransport format parameters to one or more other elements of transmitter100 as illustrated. The transport format parameters may include thespatial-frequency subcarrier modulation assignments as well as thenumber of coded bits per OFDM symbol. The transport format parametersmay also include other information to specify how the OFDM symbol is tobe modulated. In some embodiments, the transport format parameters mayinclude, in addition to the number of coded bits per OFDM symbol, thenumber of bits to be modulated on each spatial stream. In someembodiments, interleaver 104 may also be provided the subcarriermodulation assignments, although the scope of the invention is notlimited in this respect.

In some embodiments, parser 106 may parse a block of bits representingan OFDM symbol into groups having a variable number of coded bits, andsubcarrier modulators 108 may individually modulate the groups of bitson OFDM subcarriers in accordance with the spatial-frequency subcarriermodulation assignments to generate symbol-modulated subcarriers 109.IFFT circuitry 110 may generate time domain waveforms from thesymbol-modulated subcarriers for subsequent RF transmission over thespatial channels. In these embodiments, the number of groups of bits maybe equal to a number of spatial channels multiplied by a number of theOFDM subcarriers of the channel.

In some embodiments, transmitter 100 may include an RF chain for eachspatial channel. The RF chain may comprise one of RF circuitry 112 andan associated one of IFFT circuitry 110 for each spatial channel.Although one of antennas 114 is illustrated for each RF chain, this isnot a requirement. Modulators 108, on the other hand, may be associatedwith particular subcarriers rather than spatial channels so that any onemodulator may modulate corresponding subcarriers (i.e., of the samesubcarrier frequency) of each of the spatial channels. For eachsubcarrier, multiple symbols may be processed by one of modulators 108.

In some embodiments, where there are N OFDM subcarriers and M spatialchannels, parser 106 may provide N×M groups of bits. In someembodiments, N and M may be positive integers less than 100. In someexample embodiments in which there are forty-eight data subcarriers andten spatial channels, there may be up to 480 groups of bits. Each groupof bits, for example, may have up to six bits each when a maximummodulation of 64 QAM is used, although the scope of the presentinvention is not limited in this respect.

In some embodiments, parser 106 may be a spatial-frequency parser toparse a block of bits of a variable size into spatial-frequency groupsof bits. Each spatial-frequency group may be associated with a spatialchannel and a subcarrier frequency of the associated spatial channel.

In some embodiments, the functions of interleaver 104 and parser 106 maybe performed in a different order than described above. For example, theparsing may be performed before interleaving, although the scope of theinvention is not limited in this respect. In these embodiments, a symbolinterleaver may be used after parsing. In some embodiments, interleavingmay be performed separately for each spatial channel, although the scopeof the invention is not limited in this respect. In some embodiments,encoder 102 may use a code, such as a low-density parity check code(LDPC) that does not always require an interleaving operation.

In some embodiments, transmitter 100 may comprise a spatial-frequencyinterleaver. In these embodiments, the functions of interleaver 104 andparser 106 may be combined into the spatial-frequency interleaver. Inthese embodiments, interleaving may be performed before, during or afterparsing, and may be performed on any groups of bits to help assure thatadjacent bits are separated by at least two subcarriers.

In some embodiments, transmitter 100 may transmit an OFDM symbol on awideband communication channel. The wideband channel may comprise one ormore subchannels. The subchannels may be frequency-division multiplexed(i.e., separated in frequency) and may be within a predeterminedfrequency spectrum. The subchannels may include more than one spatialchannel and the spatial channels of a particular subchannel may use thesame set of orthogonal subcarriers. In some embodiments, a widebandchannel may comprise up to four or more subchannels, and each subchannelmay have up to forty-eight or more orthogonal data subcarriers. Eachsubchannel may have a number of spatial channels determined by thenumber of RF chains. In some of these embodiments, the subchannels mayhave bandwidths of approximately 20 MHz, and each OFDM subcarrier ofeach spatial channel of a subchannel may be assigned an individualspatial-frequency subcarrier modulation assignment between zero and tenor more bits per symbol. In these embodiments, transmitter 100 maytransmit the OFDM symbol over the spatial channels of the subchannelsthat comprise the wideband channel. Subchannels with greater or lesserbandwidths may also be suitable.

In some embodiments, the frequency spectrums for a channel may compriseeither a 5 GHz frequency spectrum or a 2.4 GHz frequency spectrum. Inthese embodiments, the 5 GHz frequency spectrum may include frequenciesranging from approximately 4.9 to 5.9 GHz, and the 2.4 GHz spectrum mayinclude frequencies ranging from approximately 2.3 to 2.5 GHz, althoughthe scope of the invention is not limited in this respect, as otherfrequency spectrums are also equally suitable.

FIG. 2 is a block diagram of a receiver in accordance with someembodiments of the present invention. Receiver 200 may be part of awireless communication device and may receive OFDM communication signalsover an OFDM channel having more than one spatial channel. The OFDMsignals may have been transmitted in accordance with ABL scheme whichemploys an individual spatial-frequency subcarrier modulation assignmentfor each subcarrier of each spatial channel. Transmitter 100 (FIG. 1) isan example of a transmitter that may transmit suitable OFDM symbols,although other transmitters may also be suitable.

Receiver 200 may comprise one or more of spatially diverse antennas 214and RF circuitry 212 to receive an OFDM symbol over a plurality ofspatial channels. Receiver 200 may also comprise fast Fourier transform(FFT) circuitry 210 to generate frequency domain representations 209 ofthe OFDM symbol received over the OFDM subcarriers. Receiver 200 mayalso comprise subcarrier demodulators 208 to demodulate frequency domainrepresentations 209 for each subcarrier from each of the spatialchannels in accordance with the spatial-frequency subcarrier modulationassignments to generate groups of bits 207. Receiver 200 may alsocomprise deparser 206 to combine groups of bits 207 to generate blocksof coded bits 205 representing the OFDM symbol. Deinterleaver 204 mayperform a deinterleaving operation on block of coded bits 205 anddecoder 202 may decode the blocks of bits to generate decoded bitsequence 201.

In some embodiments, receiver 200 may include an RF chain for eachspatial channel. The RF chain may comprise one of RF circuitry 212 andan associated one of FFT circuitry 210 for each spatial channel. In someembodiments, antennas 214 may be spatially diverse antennas and each maybe associated with a spatial channel. Although one of antennas 214 isillustrated for each RF chain, this is not a requirement. Demodulators208, on the other hand, may be associated with particular subcarriersrather than spatial channels so that any one demodulator may demodulatecorresponding subcarriers (of the same subcarrier frequency) of each ofthe spatial channels. For each subcarrier, multiple symbols may beprocessed by one of demodulators 208.

Demodulators 208 may be implemented in various ways. In someembodiments, demodulators 208 may be implemented with a minimum meansquare error (MMSE) receiver. In other embodiments, demodulators 208 maybe implemented with a successive interference cancellation algorithm. Inyet other embodiments, demodulators 208 may be implemented or withmaximum likelihood (ML) demodulators, or soft-output ML-likedemodulators (such as difference-min-difference demodulation) witheither full or reduced search space algorithms such as sphere decoding.In some embodiments, demodulators 208 may deliver soft bit levellog-likelihood ratios (LLRs) that may be subsequently de-interleaved anddelivered to decoder 202, although the scope of the present invention isnot limited in these respects.

In some embodiments, receiver 200 may provide spatial-frequencysubcarrier modulation assignments to a transmitting station for use intransmitting OFDM symbols to receiver 200. In these embodiments,receiver 200 may further comprise a subcarrier modulation assignmentgenerator to determine the spatial-frequency subcarrier modulationassignments based on channel characteristics for each of the orthogonalfrequency division multiplexed subcarriers associated with the spatialchannels. The channel characteristics may comprise a signal to noise andinterference ratio (SINR) measured by receiver 200 from the spatialchannels, although the scope of the present invention is not limited inthis respect.

In some embodiments, a receiving station, such as receiver 200, maymeasure a multiple-input, multiple-output (MIMO) channel for eachsubcarrier. The MIMO channel may comprise a plurality of spatiallydiverse paths. The receiving station may use these measurements tocalculate a spatial subchannel SINR for each subcarrier. In theseembodiments, the receiving station may use a matrix channel comprising achannel term for each transmit-receive antenna pair and may use thematrix to calculate a SINR for each subcarrier, although the scope ofthe present invention is not limited in this respect.

In some embodiments, transmitter 100 (FIG. 1) and/or receiver 200 maytransmit and/or receive RF signals in accordance with specificcommunication standards, such as the IEEE 802.11(a), 802.11(b),802.11(g/h) and/or 802.16 standards for wireless local area network(WLAN) communications, although transmitter 100 and/or receiver 200 mayalso be suitable to transmit and/or receive communications in accordancewith other techniques. In some embodiments, the RF signals may compriseOFDM signals comprising a plurality of symbol-modulated subcarriers ineither a 5 GHz frequency spectrum or 2.4 GHz frequency spectrum.

In some embodiments, transmitter 100 (FIG. 1) and/or receiver 200 may bepart of a wireless communication device. The wireless communicationdevice may be a personal digital assistant (PDA), a laptop or portablecomputer with wireless communication capability, a web tablet, awireless telephone, a wireless headset, a pager, an instant messagingdevice, an MP3 player, a digital camera, an access point or other devicethat may receive and/or transmit information wirelessly.

Although transmitter 100 (FIG. 1) and receiver 200 are illustrated ashaving several separate functional elements, one or more of thefunctional elements may be combined and may be implemented bycombinations of software-configured elements, such as processingelements including digital signal processors (DSPs), and/or otherhardware elements. For example, some elements may comprise one or moremicroprocessors, DSPs, application specific integrated circuits (ASICs),and combinations of various hardware and logic circuitry for performingat least the functions described herein.

When transmitter 100 (FIG. 1) and receiver 200 (FIG. 2) comprise awireless communication device, there is no requirement that the numberof spatially diverse antennas used for transmission be equal to thenumber of spatially diverse antennas used for reception. In someembodiments, one set of spatially diverse antennas may be used by thewireless communication device for both reception and transmission.However, in accordance with some embodiments, transmitter 100 (FIG. 1)may include an RF chain for each spatial channel used to transmit, andreceiver 200 (FIG. 2) may include an RF chain for each spatial channelused to receive. In some embodiments, receiver 200 may includebeamforming circuitry to receive spatial channels through a singlereceive antenna, or through more than one receive antennas that do notnecessarily correspond to the number of spatial channels.

FIG. 3 is a block diagram of a wireless communication device inaccordance with some embodiments of the present invention. Wirelesscommunication device 300 may comprise transceiver 302, data processor304, spatially diverse transmit antennas 306 and spatially diversereceive antennas 308. Data processor 304 may generate a bit stream, suchas bit stream 101 (FIG. 1), for transmission by a transmitter portion oftransceiver 202. Data processor 304 may also receive a decoded bitstream, such as bit stream 201 (FIG. 2), from a receiver portion oftransceiver 302.

Wireless communication device 300 may operate in an OFDM system and mayemploy multiple antennas 306 and 308 to communicate separate datastreams on spatial channels. In some embodiments, each spatial channelmay use the same set of OFDM subcarriers (for receiving and/ortransmitting) and may take advantage of the differing multipathcharacteristics of the spatial channels allowing the communication ofadditional data without an increase in frequency bandwidth. Inaccordance with some embodiments, spatial-frequency subcarriermodulation assignments may be dynamically assigned on a per subcarrierbasis per spatial channel to help maximize the data-carrying capacity ofthe channel.

In some embodiments, transmitter 100 (FIG. 1) and receiver 200 (FIG. 2)may be suitable for use as transceiver 302, although other transceiversmay also be suitable. In some of these embodiments, the transmitter andreceiver may share IFFT and FFT circuitry. In some embodiments,transceiver 302 may operate in a time-division duplex mode and employ asingle set of spatially diverse antennas for use in both transmittingand receiving, although the scope of the invention is not limited inthis respect.

In some embodiments, transceiver 302 may include a transmit antennabeamformer to perform beamforming on the time-domain waveforms forsubsequent RF transmission over the spatial channels with a singletransmit antenna, or a plurality of transmit antennas, such as one ormore of antennas 306 . In some embodiments, transceiver 302 may includea receive antenna beamformer to perform beamforming on the time-domainwaveforms for subsequent RF transmission over the spatial channels witha single receive antenna, or a plurality of receive antennas, such asone or more of antennas 308. In these embodiments, antennas 308 andantennas 306 are not necessarily associated with spatial channels.

Wireless communication device 300 is illustrated as a MIMO system. Inthese embodiments, wireless communication device may employ more thanone transmit antenna for more than one output data path and more thanone receive antenna for more than one input data path. In otherembodiments, wireless communication device 300 may use a single one oftransmit antennas 306, and more than one spatially diverse receiveantennas 308. In other embodiments, wireless communication device 300may use single one of receive antennas 308, and more than one spatiallydiverse transmit antennas 306.

In some embodiments, a single transmit antenna may be used to transmitover more than one spatial channel by employing beamforming and/orbeam-steering techniques. In these embodiments, the transmitter portionof transceiver 302 may include an RF chain associated with each spatialchannel. In some embodiments, a single receive antenna may be used toreceive over more than one spatial channel by employing beamformingand/or beam-steering techniques. In these embodiments, the receiverportion of transceiver 302 may also include an RF chain associated witheach spatial channel.

In some embodiment, reception and transmission may be performed by thesame one or more antennas, suitable diplexing or signal separationcircuitry may be used to separate received and transmitted signals.

In some embodiments, when device 300 operates as part of an OFDMcommunication system, such as part of a WLAN, the transport format maybe known at both the transmitting station and the receiving station. Insome time-division duplex (TDD) embodiments, due to channel reciprocity,both ends of the link may apply the same transport format parameterselection algorithm for selection of the spatial-frequency subcarriermodulation assignments. However estimation errors may result in adifferent format being applied at the receiving station than wasactually transmitted. In other embodiments, transport format parametersmay be negotiated between the transmitting station and the receivingstation. In some embodiments, a request-to-send/clear-to-send (RTS/CTS)signaling structure may be used, although the scope of the invention isnot limited in this respect.

FIG. 4 is a flow chart of an OFDM symbol transmission procedure inaccordance with some embodiments of the present invention. OFDM symboltransmission procedure 400 may be performed by a transmitter, such astransmitter 100 (FIG. 1) to generate an OFDM symbol and transmit theOFDM symbol on more than one spatial channel in accordance withspatial-frequency subcarrier modulation assignments, although othertransmitters may also be suitable.

In operation 402, a block of coded bits representing an OFDM symbol maybe interleaved. The OFDM symbol may comprise a number of coded bits perOFDM symbol (Ncbps) 403 which may be determined by the spatial-frequencysubcarrier modulation assignments for each subcarrier for each spatialchannel. In some embodiments, the number of spatial-frequency subcarriermodulation assignments may equal the number of subcarriers multiplied bythe number of spatial channels. In some embodiments, operation 402 maybe performed by bit interleaver 104 (FIG. 1), although the scope of theinvention is not limited in this respect.

In operation 404, the bits may be parsed into groups representingsymbols. In some embodiments, the parsing of operation 404 may beperformed prior to the interleaving of operation 402. In theseembodiments, interleaving may be performed on each parsed group of bits,although the scope of the invention is not limited in this respect. Thenumber of bits per individual group may be based on a spatial-frequencysubcarrier modulation assignment for each subcarrier for an associatedspatial channel. The number of groups may be equal to the number ofsubcarriers multiplied by the number of spatial channels. In someembodiments, operation 404 may be performed by parser 106 (FIG. 1),although the scope of the invention is not limited in this respect.

In operation 406, the groups of bits representing symbols are modulatedonto OFDM subcarriers to generate symbol-modulated carriers for eachspatial channel. The modulation may be based on the spatial-frequencysubcarrier modulation assignments for each subcarrier for an associatedspatial channel. In some embodiments, operation 406 may be performed bysubcarrier modulators 108 (FIG. 1), although the scope of the inventionis not limited in this respect. Modulators 108 (FIG. 1) may beassociated with individual subcarriers.

In operation 408, a time-domain waveform may be generated for eachspatial channel. The time domain waveform may be generated from all OFDMsubcarriers associated with a spatial channel. In some embodiments,operation 408 may be performed by IFFT circuitry 110 (FIG. 1), althoughthe scope of the invention is not limited in this respect. In theseembodiments, a modulator may be provided for each subcarrier, and anIFFT processor may be provided for each spatial channel.

In operation 410, the OFDM symbol comprising the time domain waveformsmay be transmitted over the spatial channels Operations 410 may beperformed by RF circuitry 112 (FIG. 1) with one or more of antennas 114(FIG. 1), although the scope of the invention is not limited in thisrespect. In some embodiments, operations 408 and 410 may comprisegenerating RF signals with an RF chain associated with each spatialchannel. The RF chain, for example, may comprise one of RF circuitry 112(FIG. 1) and an associated one of IFFT circuitry 110 (FIG. 1) for eachspatial channel. In some embodiments, each RF chain may include anantenna for transmitting RF signals associated with a correspondingspatial channel, although the scope of the preset invention is notlimited in this respect.

In other embodiments, a single antenna or another number of antennas notnecessarily related to the number of spatial channels may be used. Inthese embodiments, operation 410 may include performing beamformingoperations on the outputs of RF circuitry 112 (FIG. 1) to allow thetransmission of the RF signals on each spatial channel on a singleantenna of other number of antennas.

FIG. 5 is a flow chart of an OFDM symbol reception procedure inaccordance with some embodiments of the present invention. OFDM symbolreception procedure 500 may be performed by a receiver, such as receiver200 (FIG. 2), to receive an OFDM symbol on more than one spatial channelthat has been transmitted in accordance with spatial-frequencysubcarrier modulation assignments.

In operation 502, an OFDM symbol may be received over a plurality ofspatial channels. In some embodiments, a spatial channel may beassociated with a spatially diverse antenna. In other embodiments, thespatial channels may be received using a single antenna or other numberof antennas not necessarily related to the number of spatial channels.Operation 502 may include converting received RF signals to serialsymbol streams for each spatial channel. In some embodiments, operation502 may be performed by RF circuitry 212 (FIG. 2), although the scope ofthe invention is not limited in this respect.

In operation 504, frequency domain representations are generated foreach spatial channel. In some embodiments, FFT circuitry 210 (FIG. 2)may perform operation 504, although the scope of the invention is notlimited in this respect.

Operation 506 demodulates the frequency domain representations based ona spatial-frequency subcarrier modulation assignment associated witheach subcarrier and each spatial channel. Operation 506 may alsogenerate a group of bits from each subcarrier received over each spatialchannel based on the spatial-frequency subcarrier modulationassignments. In some embodiments, operation 506 may be performed bydemodulators 208 (FIG. 2), although the scope of the invention is notlimited in this respect.

In some embodiments, operations 502 and 504 may be performed by an RFchain for each spatial channel. The RF chain may, for example, compriseone of RF circuitry 212 (FIG. 2) and an associated one of FFT circuitry210 (FIG. 2) for each spatial channel. Although one of antennas 214(FIG. 2) is illustrated for each RF chain, this is not a requirement.Demodulators 208 (FIG. 2) may perform operation 506 and may beassociated with particular subcarriers rather than spatial channels sothat a demodulator may demodulate corresponding subcarriers (i.e., ofthe same subcarrier frequency) of each of the spatial channels.

In some embodiments, each RF chain may include an antenna for receivingRF signals associated with a corresponding spatial channel, although thescope of the preset invention is not limited in this respect. In someother embodiments, a single antenna or another number of antennas notnecessarily related to the number of spatial channels may be used. Inthese other embodiments, operation 502 may include performingbeamforming operations on either the inputs to RF circuitry 212 (FIG. 2)to allow the separation of the RF signals for each spatial channel.

Operation 508 may combine the groups of bits in a proper order based onthe spatial-frequency subcarrier modulation assignments to generate ablock of bits. In some embodiments, operation 508 may be performed bydeparser 206 (FIG. 2), although the scope of the invention is notlimited in this respect.

Operation 510 may deinterleave the block to generate a number of codedbits representing the OFDM symbol. Operation 510 may be performed bydeinterleaver 204 (FIG. 2), although the scope of the invention is notlimited in this respect. The block may be subsequently decoded togenerate a coded bit stream. In some embodiments, the parsing may beperformed before deinterleaving.

Although the individual operations of procedure 400 (FIG. 4) andprocedure 500 are illustrated and described as separate operations, oneor more of the individual operations may be performed concurrently, andnothing requires that the operations be performed in the orderillustrated.

Embodiments of the invention may be implemented in one or a combinationof hardware, firmware and software. Embodiments of the invention mayalso be implemented as instructions stored on a machine-readable medium,which may be read and executed by at least one processor to perform theoperations described herein. A machine-readable medium may include anymechanism for storing or transmitting information in a form readable bya machine (e.g., a computer). For example, a machine-readable medium mayinclude read-only memory (ROM), random-access memory (RAM), magneticdisk storage media, optical storage media, flash-memory devices,electrical, optical, acoustical or other form of propagated signals(e.g., carrier waves, infrared signals, digital signals, etc.), andothers.

The Abstract is provided to comply with 37 C.F.R. Section 1.72(b)requiring an abstract that will allow the reader to ascertain the natureand gist of the technical disclosure. It is submitted with theunderstanding that it will not be used to limit or interpret the scopeor meaning of the claims.

In the foregoing detailed description, various features are occasionallygrouped together in a single embodiment for the purpose of streamliningthe disclosure. This method of disclosure is not to be interpreted asreflecting an intention that the claimed embodiments of the subjectmatter require more features than are expressly recited in each claim.Rather, as the following claims reflect, invention lies in less than allfeatures of a single disclosed embodiment. Thus the following claims arehereby incorporated into the detailed description, with each claimstanding on its own as a separate preferred embodiment.

1. A multiple-input-multiple output (MIMO) transmitter for transmittingan orthogonal frequency division multiplexed (OFDM) symbol using aplurality of antennas comprising: subcarrier modulators to individuallymodulate groups of a variable number of coded bits on OFDM subcarriersin accordance with spatial-frequency subcarrier modulation assignmentsto generate symbol-modulated subcarriers; circuitry to combinecorresponding symbol-modulated subcarriers for transmission on each of aplurality of spatial channels, the spatial channels being non-orthogonaland having different multipath characteristics; and a beamformer toperform beamforming on the combined symbol-modulated subcarriers fortransmission over the spatial channels, wherein the spatial-frequencysubcarrier modulation assignments are provided by a receiver anddetermined from spatial channel characteristics of the spatial channelsprior to transmission.
 2. The MIMO transmitter of claim 1 furthercomprising a spatial-frequency parser to parse a block of bitsrepresenting the OFDM symbol into spatial-frequency groups of thevariable number of coded bits, each spatial frequency group beingassociated with a spatial component and a frequency component of theOFDM symbol, the spatial component being associated with one of thespatial channels, the frequency component being associated with one of aplurality of the OFDM subcarriers.
 3. A multiple-input-multiple output(MIMO) receiver for receiving an orthogonal frequency divisionmultiplexed (OFDM) symbol comprising: a beamformer to performbeamforming on signals received through a single antenna that weretransmitted by a MIMO transmitter over a plurality of spatial channels,the beamformer to help separate signals of the spatial channels, thespatial channels being non-orthogonal and having different multipathcharacteristics; circuitry to determine spatial-frequency subcarriermodulation assignments from channel characteristics of the spatialchannels for transmission to the MIMO transmitter; and subcarrierdemodulators to demodulate received OFDM subcarriers that comprise theOFDM symbol based on the spatial-frequency subcarrier modulationassignments.
 4. The MIMO receiver of claim 3 wherein the beamformerperforms beamforming on signals that are received through each of aplurality of single antennas, the signals having been transmitted by theMIMO transmitter over the plurality of spatial channels.